1. Field of the Invention
The present invention generally relates to a magnetic head and a magnetic storage device using a recording medium. More particularly, the present invention relates to a magnetic head for expanding the track width of the recording medium, a control circuit for controlling the magnetic head, and a storage device that uses the magnetic head and the control circuit.
2. Description of the Related Art
The development of magnetic storage devices, including a hard disk drive (HDD), is being advanced to achieve higher recording densities, and their recording density is increasing at an annual rate of 30 to 100%. In HDDs being mass-produced, there has already been realized an areal recording density of 100 Gb/in2.
Note here that the areal recording density of a magnetic storage device is determined by the product of bits per inch (BPI) and tracks per inch (TPI). In other words, the areal recording density is determined by the product of BPI denoting bit density in the track direction of a recording medium and TPI denoting bit density in the cross-track direction thereof. BPI is mainly dominated by the amount of noise in the magnetization reversal region of a recording medium. Accordingly, development efforts are being made to reduce the noise of the recording media, so as to be able to increase the density of BPI. Specifically, the development efforts are being made to reduce a residual magnetization-thickness product and increase coercivity since it is well known that the amount of noise in a recording medium is proportional to the residual magnetization-thickness product and is inversely proportional to the coercivity. Note here that although the amount of noise decreases as the residual magnetization-thickness product is reduced, there arises the problem that read signal output becomes smaller. In connection with this problem, the techniques called CPP-GMR (Current Perpendicular to Plane Giant Magneto Resistance) and TuMR (Tunneling Magneto Resistance) that overwhelmingly surpass the magneto-resistive effect rate of GMR (Giant Magneto Resistance), which is the related art, have already been employed in magnetic heads for mass-production since it is effective to increase the magneto-resistive effect rate of a read element of the magnetic head.
In addition, with regard to the increase of coercivity, there is the constraint that it is only possible to increase coercivity to the extent of causing magnetization reversal at the write magnetic field of a write element of the magnetic head. With regard to the write magnetic field intensity of a write element portion, a magnetic material having a magnetic flux density of 2.45 T, which is considered as a physical limit, has already been employed in magnetic heads for mass-production and, therefore, it is difficult to further increase the write magnetic field intensity. Note that failure to fully reverse magnetization at the write magnetic field of the write element is undesirable since magnetic crystal grains, the magnetization of which has not been reversed, serve as a noise source. In connection with this problem, there has been proposed a method wherein a heat source, such as a laser, is disposed in a magnetic head or in a head slider whereon the magnetic head is mounted, in order to radiate laser light at a recording medium when writing (recording) thereto, thereby temporarily decreasing the coercivity of the recording medium. Use of this mechanism makes it possible to attain large coercivity without being obliged to adhere to the write magnetic field intensity of the write head element portion. In this way, the densification of BPI has been achieved by a variety of technical approaches.
On the other hand, it is difficult to control TPI for reasons of the characteristics of the recording media and, therefore, the densification has been attempted by reducing the widths of the read and write elements of the magnetic head in the cross-track direction thereof, i.e., so-called core widths. The core width of the write element is designed to be larger than that of the read element so that the read head element does not sense the track edge noise of the recording medium. For this reason, the machining accuracy of the read element is required to be higher than that of the write element. Note here that the core width of the read element was as extremely large as approximately 2 μm for an areal recording density of 2 Gb/in2, whereas it is now as extremely small as approximately 100 nm for an areal recording density of 100 Gb/in2. A decrease in the core width means that tolerances for the dimensions of core width become even more stringent. Whereas the core width and tolerance of write elements for an areal recording density of approximately 100 Gb/in2 currently in mass-production is approximately 185±50 nm, the core width and tolerance of the read element is required to satisfy machining accuracy as extremely stringent as approximately 100±10 nm. The read and write elements of the magnetic head are fabricated after being subjected to a series of complex processes wherein a process based on a photolithography technique for machining the elements into desired shapes, a plating process for depositing metal films, a sputtering process, and a polish process based on chemical mechanical polish (CMP) are repeated over and over again. For this reason, it becomes more difficult to make the magnetic head fall within the specified tolerances thereof in a manufacturing process as the core width becomes narrower. Although the degradation of read signal output is caused if the read element core width is narrower than the tolerable limit thereof, there is no major problem as long as CPP-GMR or TuMR elements having extremely high magneto-resistive effect rates are used. If the core width is wider than the tolerable limit thereof, however, there arises the problem that the read element senses track edge noise. It is therefore an object of the present invention to solve the problem of preventing track edge noise from being sensed even if the core width of the read element of the magnetic head is wider than the tolerable limit thereof. It is another object of the present invention to provide a control circuit for controlling the magnetic head.